Cellular networks generally comprise a series of base stations arranged to allow mobile stations within range of the network to contact other mobile stations and be able to access the public switched telephone network (PSTN). The base stations maintain wireless communication links with nearby mobile stations, but generally require hard infrastructure in the form of wiring to link the base stations to other parts of the cellular network and to outside networks and backbone networks. The laying of cable to provide such infrastructure is expensive and becomes more expensive if the infrastructure is to include any kind of backup provision. The cost of wired infrastructure is one of the limiting factors in extending a cellular network to remote areas.
The telephony system including cellular networks and the PSTN, is generally based on the E1, or possibly T1, protocol for multiplexing transmissions into time slots. The protocol is strongly synchronous in that the individual transmission to which a time slot is assumed to belong to is determined from its temporal position amongst the other time slots. Thus an individual transmission which does not have current data creates blank slots to reserve its current position.
Much available data carrying capacity is based on the TCP/IP protocol, which involves individual data packets being sent out over a network in accordance with destination information contained in a packet header. A single transmission is thus broken down into numerous packets which are each sent out independently over the network. The packets may be sent along different routes depending on availability and may not arrive in the order in which they have been sent. However the packet headers may be used by the receiving application to rebuild an original sequence from the packets.
The E1 (and T1) protocol thus depends on the preservation of a temporal relationship between time slots whereas the TCP/IP protocol does not preserve timing information. Thus TCP/IP based capacity cannot be used to transport E1 data since synchronization is not preserved, rendering the E1 datastream irrecoverable.